Non-magnetic austenitic steel



\ nickel.

tively 10w alloy United States Patent New Jersey No Drawing. A plicatlouDecember 27, 1957 Se No. 705,489

2 Claims. (Cl. 75-126) This invention pertains to a substantiallynickel-free, non-age hardenable and non-magnetic, austenitic .steel,which remains non-magnetic on cold working.

It is the object of this invention to provide a substantiallynon-magnetic, austenitic alloy steel practically free from nickel. Asidefrom the Hadfield manganese steel, which has limits of usefulnessbecause of fabrication difilculties, all steels now available which arerelatively non-magnetic, that is, those which have permeability valuesunder about 1.2 contain appreciable amounts of Characteristic of theseare the austenitic stainless steels, for examples, types 302(approximately 18% chromium and 8% nickel) and 310 (approximately 25%chromium and 20% nickel). Another important nonmagnetic material forelectrical equipment contains about 35% carbon, 13% manganese, 8.0%nickel, balance iron.

The low carbon, 16% manganese, 16% chromium, .l% nitrogen steel which isnow becoming available as a replacement for type 302 is non-magnetic asannealed, but it becomes appreciably magnetic after it is subjected tosome cold working.

The recently developed demand for a non-magnetic, low cost steel forcertain types of naval vessels, together with the present limit onavailability of nickel for such materials, make it highly desirable toprovide a relasteel which has the desired non-magnetic which is notdependent on nickel for this I have invented a steel which meets thesepreerred" ranges of property and characteristic. requirements, the"broad and analysis of which are as follows:

Analysts, Wt. Percent Broad Preferred Balance, Iron.

For a ferrous material to be non-magnetic it is necessary that itsstructure be substantially austenitic at ambient temperatures. To retainthe austenitic structure at low temperatures, it is necessary that thesteel have 2,876,096 Patented Mar. 3, 1959 in its compositionsubstantial quantities of those elements which are elfective insuppressing its Ms temperature or austenite-to-martensite transformationtemperature. These are notably carbon, manganese, chromium and nickel ashas been established inthe publication Martensite Reactions in AlloySteels by P. Payson and C. H. Savage, Trans. A. S. M., vol. 33, 1944, p.261. Nitrogen is also known to have an appreciable stabilizing effect onaustenite in steel.

Since manganese is highly effective in suppressing the martensitereaction, and since it is one of the least expensive steel alloyingelements, I depend primarily on this element in the production of mynon-magnetic steel. Furthermore, the presence of high manganese in thesteel permits relatively large amounts of nitrogen to be dissolvedtherein which is desirable to attain the objective of a relatively lowcost non-magnetic material. I set a minimum of about 11% manganese forthe steel of this invention so that I may use relatively low carbontherein. If lower amounts of manganese are used, it is necessary also toemploy relatively high carbon in order to stabilize the austenite, andit is further desirable to keep the carbon as low as possible to assurethat the weldability of the steel will be at a maximum.

Since most of the manganese in steel is lost in the remelting of scrap,it is preferable not to depend entirely on manganese and carbon toprovide stability of the austenite, and up to this time nickel has beendepended upon to supplement the manganese in stabilizing the austenite.But I have established that chromium in amounts of about 11 to 15%serves admirably for this purpose. Ordinarily chromium is thought of asa ferrite forming element, which indeed it is when the carbon andnitrogen contents of the steel are low. But when both carbon andnitrogen in the steel are above about a total of 0.30%, a sizeablequantity of chromium, that is, well over 15%, is needed to establishfree ferrite in the steel. Another advantage in the use of chromiumrather than nickel for stabilizing the austenite in the non-magneticsteel is that chromium like manganese increases the solubility ofnitrogen in the steel. The low limit of chromium in the stablenon-magnetic steel depends on the amounts of carbon and nitrogenpresent, in addition to manganese, as will 'be shown below.

It is well known that semi-stable austenite can be made to transform toa magnetic phase by subjecting it to plastic deformation at roomtemperature. The relative magnetic susceptibility ofthe steel after coldworking is, therefore, a measure of the stability of the austenite, thelower the susceptibility with more and more cold working, the greaterthe stability of the steel.

The data in Table I below show the effects of composition on themagnetic susceptibility after cold working of the CMnCrN steelcontaining varying amounts of carbon and nitrogen in addition tochromium:

TABLE I Eflect of composition and percent cold work on magneticsusceptibility of steels [All steels were first annealed by an air cnolfrom 1950 F. and were then cold reduced as indicated] Percent RockwellPermeability Bar 0 Mn 51 Ni Cr N Cold Be CHardductlon ness H-10 H-BOO 4m.32 13.2 0.32 0.68 8.7 0.06 10 (strongl) em .29 13.1 0.32 0.68 8.6 0.1218 "ii- ,(magnet m 4325. .41 13.3 0.23 an as 0.11 10 as "ifii' 11044 N41 1.151 1.314 0 21 1.002 4323 .27 13.6 0.82 0.02 11.0 0.17 10 33 1.0181.033 a0 41 1.121 1.258 0 25 1.001 4020 .40 13.2 0.42 0.77 12.4 0.16 1034 1.001 1.008 so 41 1.035 1.001

TABLE 11 v Mechanical properties of non-magnetic C-Mn-Cr-N steel [Bar4320 1 annealed at 1950- F.]

0.2% yield strength 64,000 p. s. i. Tensile strength 136,000 p. s. i.Flnngatinn I v 65%, Red/of area 60%. V-notch Charpy at 80' F 140 ft.lbs. V-notch Charpy at minus 40' F 105 ft. lbs. V-notch Charpy at minus100 P 75 ft. lbs.

! Composltlon given in Table I.

Some applications for such non-magnetic steel are: deck-plates;structural members, such as plates, sheets, I-beams, angles, channels,etc.; or assemblies of such structural members in girders, trusses,engine foundations, fly-wheels, etc. The assemblies can be made eitherby riveting or welding.

That the composition range above specified for providing thesubstantially non-magnetic, austenitic steel of the invention is highlycritical is evidenced by the following. If the manganese is droppedappreciably below my lower limit of about 11%, while retaining C, Cr andN within my specified limits therefor, the steel nevertheless becomeshighly magnetic on quenching. Thus two steels of analysis0.36Cl2Cr-0.17N but containing 7% and 8.6% Mn, respectively, were foundto be highly magnetic on aging for 16-32 hours at 1400 F. Other steelscontaining chromium above my upper limit of 15% were found to be highlymagnetic, as solution treated or as solution treated and aged. Thus asteel analyzing about .3C19Cr-l2Mn0.l5N was found to be highly magneticon solution treating at 2100 F. and water quenched. The same was foundto be true of other steels analyzing about 0.07/0.3C-23Cr-13Mn- 0.6N,and alsov about 0.25C29Cr-10Mn0.25N, the latter, being highly magneticboth as solution treated and as subuquently aged at 1400' F. for 2 to100 hours. This same result was found to be true of steels containing 4manganese below and carbon and chromium above my range, such as steelscontaining about 0.5/0.8C24Cr-- 5Mn-0.45N, which were highly magnetic assolution treated at 2100' F. and quenched and thereafter aged 16 hoursat 1400' F. v

. The steel of my invention is not appreciably hardened by an aging heattreatment. Thus a typical steel analyzing about 0.3C--l2Cr-11Mn0.2N hada hardness of 24 Re (Rockwell "6") as solution treated at 2100 F. andwater quenched. 0n subsequent aging for 8- hours at 1400 F., thehardness had increased only to I 28 Re and to only 31 Re after hours ofaging. This steel was non-magnetic in all conditions of heat treatment.

This application is a continuation-in-part of my applications SerialNos. 349,087, tiled April 15, 1953, 414,498,- filed March 5, 1954, and421,997, filed April 9, 1954,

all abandoned.

What I claim is:

1. An austenitic steel characterized in being substantially non-agehardenable and in having a maximum magnetic permeability of 1.2 with amagnetizing force of about 500 oersteds, both as annealed and after acold reduction of about 20%, said steel consisting essentially of about:0.3 to 0.5% carbon, 11 to 15% manganese, 11 to 15% chromium, 0.15 to0.5% nitrogen, up to 1% each of nickel and silicon, and the balanceiron.

2. An austenitic steel characterized in being substantially non-agehardenable and in having a maximum magnetic permeability of 1.2 with amagnetizing force of about 500 oersteds, both as annealed and after acold reduction of about 20%, said steel consisting essentially of about:0.35 to 0.45% carbon, 13 to 14% manganese, 12 to 13% chromium, 0.15 to0.25% nitrogen, 0.2 to 0.5% silicon, 0.5 to 1% nickel, and the balanceiron.

References Cited in the file of this patent UNITED STATES PATENTS2,696,433 Tanczyn Dec. 7, 1954 2,698,785 Jennings Ian. 4, 1955 FOREIGNPATENTS 152,291 Austria Jan. 5, 1938 OTHER REFERENCES Transaction,American Society for Metals, vol. 41, pages 1306 and 1337. Published in1949 by the Society.

Making, Shaping and Treating of Steel, sixth edition, pages 12-7-1209.Edited by Camp and Farncis, published in 1951 by the United States SteelCompany, Pittsburgh, Pa.

1. AN AUSTENITIC STEEL CHARACTERIZED IN BEING SUBSTANTIALLY NON-AGEHARDENABLE AND IN HAVING A MAXIMUM MAGNETIC PERMEABILITY OF 1.2 WITH AMAGNETIZING FORCE OF ABOUT 500 OERSTEDS, BOTH AS ANNEALED AND AFTER ACOLD REDUCTION OF ABOUT 20%, SAID STEEL CONSISTING ESSENTIALLY OF ABOUT:0.3 TO 0.5% CARBON, 11 TO 15% MANGANESE, 11 TO 15% CHROMIUM, 0.15 TO0.5% NITROGEN, UP TO 1% EACH OF NICKEL AND SILICON, AND THE BALANCEIRON.